Cap for writing instrument



FIG. 1 is a front elevational view of a cap for writing instrument showing my new design, the rear elevational view being a mirror image thereof;

FIG. 2 is a right side elevational view thereof, the left side view being a mirror image thereof;

FIG. 3 is a top plan view thereof; and,

FIG. 4 is a bottom plan view thereof. 

The ornamental design for a cap for writing instrument, as shown and described. 